Antiphase OH and OI Airglow Emissions Induced By a Short-Period Ducted Gravity Wave

Numerical simulation of a ducted gravity wave event suggests that OH (8,3) and O(1S) 557.7 nm airglow emissions layers may exhibit opposite-phase intensities when perturbed by a short-period wave undergoing vertical reflection. This effect arises due to the time and temperature dependance of the OH excitation reaction, coupled with the linear polarization properties of vertically-standing waves

Breaking of Thunderstorm-Generated Gravity Waves as a Source of Short-Period Ducted Waves at Mesopause Altitudes

Numerical simulation results indicate that the breaking of atmospheric gravity waves generated by tropospheric convection can excite short-period secondary waves, which are trapped in the lower thermospheric duct and which closely resemble quasi-monochromatic structures commonly observed in airglow imaging experiments

Numerical Modeling of the Propagation of Infrasonic Acoustic Waves Through the Turbulent Field Generated by the Breaking of Mountain Gravity Waves

The nonlinear propagation of low-frequency acoustic waves through the turbulent fluctuations induced by breaking mountain gravity waves is investigated via 2-D numerical simulations of the Navier-Stokes equations, to understand the effects of atmospheric dynamics on ground-based infrasound measurements. Emphasis is placed on acoustic signals of frequency around 0.1 Hz, traveling through tens-of-kilometers-scale gravity waves and subkilometer-scale turbulence. The sensitivity of the infrasonic phases to small-scale fluctuations is found to depend on the altitudes through which they are refracted toward the Earth. For the considered cases, the dynamics in the stratosphere impact the refracting acoustic waves to a greater extent than those in the thermosphere. This work clearly demonstrates the need for accurate descriptions of the effects of atmospheric dynamics on acoustic propagation, such as here captured by the full set of fluid dynamic equations, as well as of the subsequent effects on measured signals

Accelerating Atmospheric Gravity Wave Simulations using Machine Learning: Kelvin-Helmholtz Instability and Mountain Wave Sources Driving Gravity Wave Breaking and Secondary Gravity Wave Generation

Gravity waves (GWs) and their associated multi-scale dynamics are known to play fundamental roles in energy and momentum transport and deposition processes throughout the atmosphere. We describe an initial, two-dimensional (2-D), machine learning model – the Compressible Atmosphere Model Network (CAMNet) - intended as a first step toward a more general, three-dimensional, highly-efficient, model for applications to nonlinear GW dynamics description. CAMNet employs a physics-informed neural operator to dramatically accelerate GW and secondary GW (SGW) simulations applied to two GW sources to date. CAMNet is trained on high-resolution simulations by the state-of-the-art model Complex Geometry Compressible Atmosphere Model (CGCAM). Two initial applications to a Kelvin-Helmholtz instability source and mountain wave generation, propagation, breaking, and SGW generation in two wind environments are described here. Results show that CAMNet can capture the key 2-D dynamics modeled by CGCAM with high precision. Spectral characteristics of primary and SGWs estimated by CAMNet agree well with those from CGCAM. Our results show that CAMNet can achieve a several order-of-magnitude acceleration relative to CGCAM without sacrificing accuracy and suggests a potential for machine learning to enable efficient and accurate descriptions of primary and secondary GWs in global atmospheric models

Self-Accleration and Instability of Gravity Wave Packets: 1. Effects of Temporal Localization

An anelastic numerical model is used to explore the dynamics accompanying the attainment of large amplitudes by gravity waves (GWs) that are localized in altitude and time. GW momentum transport induces mean flow variations accompanying a GW packet that grows exponentially with altitude, is localized in altitude, and induces significant GW phase speed, and phase, variations across the GW packet. These variations arise because the GW occupies the region undergoing accelerations, with the induced phase speed variations referred to as “self-acceleration.” Results presented here reveal that self-acceleration of a GW packet localized in time and altitude ultimately leads to stalling of the vertical propagation of the GW packet and accompanying two- and three-dimensional (2-D and 3-D) instabilities of the superposed GW and mean motion field. The altitudes at which these effects occur depend on the initial GW amplitude, intrinsic frequency, and degree of localization in time and altitude. Larger amplitudes and higher intrinsic frequencies yield strong self-acceleration effects at lower altitudes, while smaller amplitudes yield similar effects at higher altitudes, provided the Reynolds number, Re, is sufficiently large. Three-dimensional instabilities follow 2-D “self-acceleration instability” for sufficiently high Re. GW packets can also exhibit self-acceleration dynamics at more than one altitude because of continued growth of the GW packet leading edge above the previous self-acceleration event. --From the publisher\u27s website

Modeling of Upper Atmospheric Responses to Acoustic-Gravity Waves Generated by Earthquakes and Tsunamis

Recent studies have shown that upper atmospheric observations can be used to examine the properties of acoustic and gravity waves (AGWs) induced by natural hazards (NHs), including earthquakes and tsunamis (e.g., Komjathy et al., Radio Sci., 51, 2016). In addition to rapid processing, analysis, and retrieval of the AGW signals in data, the need remains to investigate a broad parameter space of atmospheric and ionospheric state observables for the robust constraint of coupled and nonlinear processes. Here, we present several earthquake/tsunami-atmosphere-ionosphere case studies that demonstrate the possibility to detect AGWs and constrain the characteristics of their sources. Direct numerical simulations of the triggering and wave dynamical processes, from Earth\u27s interior to the exobase, are carried out based on coupled forward seismic wave and tsunami propagation models and our state-of-the-art nonlinear neutral atmosphere and ionosphere models MAGIC and GEMINI (Zettergren and Snively, JGR, 120, 2015)

The Dynamics of Tsunamigenic Acoustic-Gravity Waves and Bathymetry Effect

The investigation of atmospheric tsunamigenic acoustic and gravity wave (TAGW) dynamics, from the ocean surface to the thermosphere, is performed through the numerical computations of the 3D compressible nonlinear Navier-Stokes equations. Tsunami propagation is first simulated using a nonlinear shallow water model, which incorporates instantaneous or temporal evolutions of initial tsunami distributions (ITD). Surface dynamics are then imposed as a boundary condition to excite TAGWs into the atmosphere from the ground level. We perform a case study of a large tsunami associated with the 2011 M9.1 Tohuku-Oki earthquake, and parametric studies with simplified and demonstrative bathymetry and ITD. Our results demonstrate that TAGW propagation, controlled by the atmospheric state, can evolve nonlinearly and lead to wave self-acceleration effects and instabilities, followed by the excitation of secondary acoustic-gravity waves (SAGWs), spanning a broad frequency range. The variations of the ocean depth result in a change of tsunami characteristics and subsequent tilt of the TAGW packet, as the wave’s intrinsic frequency spectrum is varied. In addition, focusing of tsunamis and their interactions with seamounts and islands may result in localized enhancements of TAGWs, which further indicates the crucial role of bathymetry variations. Along with SAGWs, leading long-period phases of the TAGW packet propagate ahead of the tsunami wavefront and thus can be observed prior to the tsunami arrival. Our modeling results suggest that TAGWs from large tsunamis can drive detectable and quantifiable perturbations in the upper atmosphere under a wide range of scenarios and uncover new challenges and opportunities for their observations

Die Ablösung der Grundlasten im Kanton Freiburg im 19. Jahrhundert

[1] Short-period, small-scale gravity waves are frequently observed in nighttime airglow imaging experiments. These waves are often found to be ducted and may be confined to a thin region of altitude in the mesosphere or lower thermosphere. An apparent paradox of high-altitude ducted waves is the nature of the source; it is necessary that a ducted wave be excited in situ or have been able to tunnel into the duct from another atmospheric region. In this paper, analytical and numerical solutions are presented for simple thermally ducted gravity waves that are Doppler-shifted by constant background winds. Using a continuous analytical model, duct dispersion properties are calculated for three case studies. Using a fully nonlinear numerical model, several scenarios are explored by which a tropospheric source can excite these thermally ducted wave modes. First, we validate the analytical and numerical models for the classical case of linear wave tunneling. Second, we examine the nonlinear excitation of ducted waves due to resonant wave self-interactions associated with realistic propagation and small-scale wavebreaking, for propagation in the same direction as the wind flow. Third, we consider the case of ducted wave excitation and propagation opposite to the direction to wind flow. Specifically, where horizontal group and phase velocities exhibit opposite sign in the ground-relative frame. The results suggest that ducted waves of very short period can be excited in the lower thermosphere by tropospheric sources, via simple linear and nonlinear processes. These excitation mechanisms are likely to be robust for a range of realistic thermal and thermal-Doppler ducts. Citation: Snively, J. B., and V. P. Pasko (2008), Excitation of ducted gravity waves in the lower thermosphere by tropospheric sources

Doppler Ducting of Short-Period Gravity Waves by Midaltitude Tidal Wind Structure

Multiwavelength airglow image data depicting a short-period (∼4.9 min) atmospheric gravity wave characterized by a sharp leading front have been analyzed together with synoptic meteor radar wind data recorded simultaneously from Bear Lake Observatory, Utah (41.6°N, 111.6°W). The wind data suggest the presence of a semidiurnal tide with horizontal winds peaking at around 60 m/s along the SSE direction of motion (170° from north) of this short-period wave. It was found that the gravity wave was most probably ducted because of the Doppler shift imposed by this wind structure. A marked 180° phase shift was observed between the near-infrared OH and the OI (557.7 nm) emissions. Numerical simulation results for similar ducted waves excited by idealized model sources suggest that the phase shift between the wave-modulated airglow intensities may be explained simply by chemical processes rather than by wave dynamics. Phase velocities of simulated waves, however, appear higher than those of observed waves, suggesting the importance of tidal thermal structure in determining the Doppler-ducted wave characteristics